1. Field of the Invention
This invention relates to a method and apparatus for in-flight shake testing of an aircraft fuselage.
2. Description of the Prior Art
In the development testing of both fixed wing aircraft and helicopters it is current practice to employ a shake testing technique to identify fuselage dynamic response characteristics to vibration exciting frequencies in order that suitable modifications can be incorporated to ensure that such response characteristics will be maintained within acceptable limits.
In-flight shake testing is a known test procedure for flutter substantiation on fixed wing aircraft. Such procedures can be accomplished using "bolt-on" inertia devices either electrically or hydraulically driven, and some examples of such devices and procedures are disclosed in U.S. Pat. No. 3,074,385, U.S. Pat. No. 3,552,192, U.S. Pat. No. 4,470,121 and U.S. Pat. No. 4,809,553. Traditionally, shake testing of a helicopter fuselage has been accomplished with the helicopter on the ground, and in some cases with the helicopter suspended from an overhead support attached to the rotor head to simulate a flying vehicle. One such method and apparatus for achieving such testing involves attaching external shakers and forcing vibration with random or sine swept excitations and measuring response characteristics throughout the airframe. The reasons for using this method are mainly economic although the accessibility of the fuselage for changing locations of the shakers and measuring means is another factor.
Drawbacks of such a ground based system are mainly related to nonlinearities within the fuselage which produce dynamic characteristics which vary with applied forcing and, in themselves, required detailed investigation. Furthermore, in an operational helicopter, very high loads are induced into the fuselage by main rotor torque balance and cannot be represented in ground based testing. Similarly, important operational gearbox attachment load distributions cannot be represented. These factors mean that a desire to enhance the operational characteristics of helicopters by in-depth analytical studies of fuselage dynamic characteristics are being frustrated.
An objective of this invention therefore is to provide a method and apparatus for shake testing an aircraft fuselage which overcomes these drawbacks. A further objective is to achieve that objective by providing a method and apparatus for in-flight shake testing of an aircraft fuselage.